1. Field of the Invention
The invention relates to an implant for blood vessels that has a wall comprising individual filaments combined to form a circular braiding. The implant shall in particular serve to influence the flow of blood in the area of arteriovenous malformations, for example fistulas and aneurysms. It may also be used in the treatment of ischemic strokes, for example to restore, increase or maintain the blood flow. The implant may be designed so as to be recoverable.
2. Related Art
Arteriovenous malformation may significantly impair a patient and may even result in fatal risks. In particular, this applies to arteriovenous fistulas and aneurysms, especially when these are found to exist in the cerebral region. Normally, attempts are made to close off such malformations by implants which, as a rule, are placed in position by endovascular techniques with the help of catheters.
Especially when treating aneurysms implanting platinum spirals has proven its worth, said spirals fill the aneurysm more or less completely, largely obstruct the blood inflow and enable a local thrombus or clot to form which fills and ultimately closes off the aneurysm. Nevertheless, this treatment approach only suits aneurysms that have a relatively narrow access to the vessel system, so-called aciniform aneurysms. In the event of vessel protuberances having a wide access to the blood vessel there is a risk that the implanted spirals may be flushed out and cause damage to other areas of the vascular system.
In such cases it has already been proposed to place a kind of stent into position that “bars” the opening of the aneurysm and in this way prevents occlusion spirals from being flushed out. Stents of this nature that are provided with a wide-meshed wall have certain drawbacks, however.
On the one hand, this concerns the wide-meshed structure which does not prevent blood from entering the aneurysm. So if the occlusion means does not occupy the aneurysm space adequately the pressure exerted on the vessel wall remains undiminished. An after-treatment in this case may be difficult, however, because the stent will obstruct access to the aneurysm and impair the placement of additional occlusion means.
Another drawback is that the stent cannot be adapted to its placement site. In the interest of functioning optimally the stent should have close contact with the vessel wall but not exert excessive pressure on the wall. Other than stents serving the purpose of expanding vessels to counteract stenoses this type of stent must rather be viewed as a kind of sleeve the influence of which on the vessel lumen and endothelium wall of the vessel shall be as slight as possible. It thus follows that this type of stent is only of limited use when it comes to meet the requirements in question even if it has been selected especially for the envisaged purpose.
Stents consisting of wire braiding are known for a long time, particularly for applications in the coronary area. These stents are usually manufactured as a round braiding structure with the individual wire filaments forming the stent wall in layers of oppositely running spirally or helically shaped elements. In this way a mesh braiding is produced that both supports in radial direction and is permeable to blood.
A problem encountered with these stents of circular braiding design is that the small-diameter loose ends existing on the free ends may have traumatic effects.
As proposed by U.S. Pat. No. 4,655,771 (Wallsten) such a stent of circular braiding is provided with U-shaped connecting links arranged between the loose ends which makes it atraumatic. However, the U-shaped connecting links are prone to cause stresses and thus lead to deformation of the stent.
As per U.S. Pat. No. 5,061,275 (Wallsten et al.) the loose ends of such wire stents are rounded off by laser treatment to counteract traumatization. The stent proposed in that publication also consists of circular braiding the individual wires of which are provided with impressions in the knots area so as to enable a stress-free fixation within the wall structure.
Such stents of circular braiding design consisting of filaments are, when used for the treatment of stenoses, expanded hydraulically by means of balloons at the placement site and attached to the vessel wall. During placement the balloon attached to a guide wire serves as transportation element over which the stent is crimp-mounted. However, such a transportation element should not be used for implants intended to influence or channel the flow of blood in the cerebral region; on the contrary, an implant automatically adapting to the vessel diameter and leaning against the vessel wall would be of advantage in this case.
Another problematic aspect associated with stents or implants made of wire braiding is their manufacture. Manufacturing them in the form of a braided endless hosing cut off to the desired length is viewed beneficial. In this case loose wire ends are produced at the two ends of the cut-to-size hosing which must be made blunt at great expense, for example by providing for the attachment of the above mentioned connecting links.